1. Field of the Invention
The present invention relates to a Vertical Cavity Surface Emitting Laser (VCSEL) including a laminated structure in the lamination direction and a method of manufacturing the VCSEL.
2. Description of the Related Art
A VCSEL emits light in the direction orthogonal to a substrate differently from the existing edge emitting laser diodes. In the VCSEL, many devices are able to be arranged in a state of a two dimensional array on the same substrate. Therefore, the VCSEL has recently attracted attention as a light source for a digital copy machine or a printer.
For example, as illustrated in FIG. 9, such a type of VCSEL includes a laminated structure 119 in which a lower DBR layer 111, a lower cladding layer 112, an active layer 113, an upper cladding layer 114, a current confinement layer 115, an upper DBR layer 116, and a contact layer 117 are layered from a substrate 110 side in this order on the substrate 110. In the upper section of the laminated structure 119, specifically, in the region from the upper section of the DBR layer 111 to the contact layer 117, a columnar mesa 118 is formed. Further, a circular upper electrode 120 having an aperture in the center is formed on the top face of the mesa 118, and a lower electrode 121 is formed on the rear face of the substrate 110.
Further, in the laser diode, an insulative pedestal 122 is provided in the base region of the mesa 118 in the top face of the lower DBR layer 111 for the purpose of decreasing the capacity component of an electrode pad 124 (described later). Further, an insulative protective film 123 is provided on the entire surface other than part of the upper electrode 120. The protective film 123 has an aperture 123A on the upper electrode 120. In the aperture 123A, the part of the upper electrode 120 is exposed. Further, on the surface of the protective film 123, the electrode pad 124 is formed from the pedestal 122 to the exposed portion in the aperture 123A in the upper electrode 120. For the structure of such a laser diode, for example, a description is given in Japanese Unexamined Patent Application Publication 2001-210908.
In such a laser diode, a current injected from the upper electrode 120 passes the upper DBR layer 116, is confined by the current confinement layer 115, and then reaches the active layer 113. As a result, light is emitted in the active layer 113. The light emitted in the active layer 113 is reflected and amplified by the lower DBR layer 111 and the upper DBR layer 116. After that, the light is emitted as laser light from the aperture provided in the upper electrode 120.
In the foregoing laser diode, in forming the mesa 118, it is necessary to provide etching down to a region under the active layer 113. On the surface after etching, part of the lower DBR layer 111 is exposed. As illustrated in FIG. 10, the lower DBR layer 111 is formed by alternately laminating a low-refractive index layer 111A with a high Al (aluminum) composition and a high-refractive index layer 111B with a low Al composition being λ/4 thick. FIG. 10 illustrates an enlarged view of the laminated structure 119 of FIG. 9. In general, a distribution in the thickness direction due to etching is larger than λ/4 (λ is an oscillation wavelength of the foregoing laser diode). Thus, the surface after etching is in a step-like non-flat face 111C in which the low-refractive index layer 111A and the high-refractive index layer 11B are alternatively exposed.
The low-refractive index layer 111A contains much Al, and is easily oxidized. Thus, in the case where the low-refractive index layer 111A is exposed on the surface after etching, the low-refractive index layer 111A is immediately oxidized and tarnished into black. As a result, as illustrated in FIG. 10 and FIG. 11, a circular pattern is formed from an oxidized portion in the low-refractive index layer 111A (oxidized portion 111D). The surface of the oxidized portion 111D is more roughened compared to the surface of the high-refractive index layer 111B. Thus, there has been a disadvantage that in the case where the pedestal 122 is formed on the roughened surface, the pedestal 122 is shifted from a predetermined position or is separated, and the yield is lowered.
Thus, for example, as described in Japanese Unexamined Patent Application Publication 2005-93634, the laser diode may be structured as follows. That is, an etching stop layer is provided between the lower DBR layer 111 and the lower cladding layer 112. Etching in forming the mesa 118 is stopped by the etching stop layer, and then, the etching stop layer is removed. Thereby, the base region of the mesa 118 is planarized to prevent exposure of the low-refractive index layer 111A.